1. Field
The present disclosure relates generally to wireless communication devices and, more specifically, to a method of compensating for distortion resulting from a phase-locked loop whose group delay varies with frequency.
2. Background
Offset phase-locked loops (OPLLs) are commonly used in radio frequency (RF) transmitters in order to reduce phase noise and suppress the level of noise transmitted into the receive bands of neighboring wireless channels. For example, OPLLs are used in transmitters that comply with the Global System for Mobile communication (GSM) standard. One difference between a phase-locked loop (PLL) and an OPLL is that an OPLL has an offset downconverter mixer as opposed to a divider in the feedback loop. An OPLL converts phase-modulated intermediate frequency (IF) signals to higher-frequency output RF signals. The phase modulation of the IF signal is reproduced at a higher frequency on the carrier RF signal output by the OPLL.
An OPLL exhibits characteristics of a low-pass filter. Higher-frequency noise is filtered out, while lower-frequency phase modulation is passed onto the output RF signal. As the loop bandwidth of the OPLL increases, more noise is transmitted at offset frequencies. There is a tradeoff, however, between the reduction in transmitted noise and an increase in phase error. If the bandwidth of the phase modulation received by the OPLL approaches the bandwidth of the OPLL, some of the phase modulation will also be filtered out, thereby increasing phase error. Wireless communications standards specify both a maximum phase error and a maximum amount of noise that may be emitted at predefined offset frequencies from the carrier frequency. As the loop bandwidth increases, noise is transmitted at greater offset frequencies. Thus, there is a usable loop bandwidth for which both the phase error and the transmitted noise do not exceed the specified thresholds.
FIG. 1 (prior art) shows the usable loop bandwidth of an OPLL employed in a GSM transceiver. A curve 10 shows that the noise level transmitted from the OPLL increases as the loop bandwidth increases beyond 1 MHz. A curve 11 shows that the phase noise increases sharply as the loop bandwidth decreases below 1 MHz. Thus, a usable bandwidth of about 1.6 MHz exists in the offset frequency ranges where both the phase error and the transmitted noise fall below the maximum thresholds specified by the GSM standard. For example, the transmitted noise must be less than −165 dBc/Hz and the phase noise must be less than 2 degrees rms. For additional information on employing an OPLL in a GSM transceiver, see Yamawaki et al, “A 2.7-V GSM RF Transceiver IC,” IEEE Journal of Solid-State Circuits, Vol. 32, No. 12, December 1997. Whereas GSM uses only phase modulation, both phase modulation and amplitude modulation are used by other wireless standards, such as the Enhanced Data rates for GSM Evolution (EDGE) standard and standards based on code division multiple accessing (CDMA).
OPLLs are also used in polar RF transmitters that comply with the EDGE and CDMA standards. Polar RF transmitters perform both amplitude and phase modulation and process amplitude and phase signals separately. Designing polar transmitters that can meet the modulation fidelity requirements of the EDGE and CDMA standards by achieving the specified phase error and noise thresholds, however, is rendered more difficult by the combination of phase and amplitude modulation. When amplitude modulation is applied, the effects of phase error are more serious and result in increased spectral regrowth. Moreover, the EDGE and CDMA standards employ higher frequency modulation schemes that require larger rather than smaller loop bandwidths. Uncorrupted phase modulation is achieved with higher frequency modulation by increasing the loop bandwidth. Increasing the bandwidth of the OPLL to accommodate the faster phase modulation, however, can cause the noise thresholds specified in the EDGE and CDMA standards to be exceeded. Depending on the stringency of the wireless standard, some polar transmitters have no usable loop bandwidth where both phase error and transmitted noise fall below the specified thresholds.
FIG. 2 shows the relationship between loop bandwidth, transmitted noise and spectral regrowth in a polar RF transmitter with no usable loop bandwidth under the EDGE standard. Spectral regrowth is referred to as ORFS (output RF spectrum) in EDGE systems. A curve 12 shows that the ORFS in the output signal of the polar transmitter falls below the acceptable ORFS threshold only for loop bandwidths above 1 MHz. A curve 13 shows that transmitted noise from the polar transmitter falls below the acceptable noise threshold only for loop bandwidths less than 1 MHz. Thus there is no usable loop bandwidth for which both the ORFS and the transmitted noise fall below the maximum thresholds specified by the EDGE standard.
One method of increasing the usable modulation bandwidth involves dual-point modulation. Dual-point modulation allows an OPLL faithfully to reproduce high frequency modulation without passing excessive noise. In dual-point modulation, the modulation bandwidth and the bandwidth of the loop of the OPLL are decoupled by separately modulating the OPLL input signal (the reference signal received by the phase detector) and the control signal to the voltage controlled oscillator (VCO). Thus, the frequency output by the VCO is not controlled entirely by the signal output by the loop filter of the OPLL. The carrier RF signal can therefore be modulated without being restricted by the loop bandwidth and the OPLL dynamics. A smaller bandwidth of the OPLL can be maintained that does not pass excessive noise, while higher frequency modulation can be faithfully reproduced in the output RF signal. For additional information on dual-point modulation, see Neurauter et al., “GSM 900/DCS 1800 Fractional-N Modulator with Two-Point-Modulation,” IEEE MTT-S, International Microwave Symposium 2002, pp. 425-428, and Hunter et al., “Using Two-Point Modulation to Reduce Synthesizer Problems When Designing DC-Coupled GMSK Modulators,” MX-COM, Inc., Winston-Salem, N.C., December 2002, 12 pages.
Achieving acceptable performance from polar transmitters that employ dual-point modulation can be difficult, however, because dual-point modulation is sensitive to the gain of the two paths of the modulation. The loop gain of the OPLL must be precisely matched to the gain of the path of the modulated reference signal received by the phase detector. The matching is difficult because each component of the loop contributes to the loop gain. The gain of the VCO, for example, is particularly sensitive to temperature and part-to-part deviations. The modulation fidelity of the polar transmitter degrades as the gains of the two modulation paths diverge.
A polar transmitter is sought that does not require the precise matching of gain in multiple modulation paths, but yet that accommodates faster phase modulation with a wide usable loop bandwidth without transmitting noise above the thresholds specified in the EDGE and CDMA standards. In addition, a method is sought for faithfully reproducing higher frequency modulation using an OPLL without passing excessive noise through the OPLL.